1. Field of the Invention
This invention relates to processes for producing electron deficient olefins, such as 2-cyanoacrylates, using an iminium salt, and if desired contacting the reaction byproduct with alkali to generate an amine and separating that amine therefrom.
2. Brief Description of Related Technology
Cyanoacrylate adhesives are known for their fast adhesion and ability to bond a wide variety of substrates. They are marketed as “super glue” type adhesives. They are useful as an all-purpose adhesive since they are a single component adhesive, very economical as only a small amount will do, and generally do not require any equipment to effectuate curing.
Traditionally, cyanoacrylate monomers have been produced by way of a Knoevenagel condensation reaction between a formaldehyde precursor, such as paraformaldehyde, and an alkyl cyanoacetate with a basic catalyst. During the reaction, cyanoacrylate monomer forms and polymerises in situ to a prepolymer. The prepolymer that is subsequently thermally cracked or depolymerised, yielding cyanoacrylate monomer. This approach has remained essentially the same over time, though various improvements and variants have been introduced. See e.g. U.S. Pat. Nos. 6,245,933, 5,624,699, 4,364,876, 2,721,858, 2,763,677 and 2,756,251.
In U.S. Pat. No. 3,142,698, the synthesis of difunctional cyanoacrylates using a Knoevenagel condensation reaction is described. However, the ability to thermally depolymerise the resulting, now crosslinked, prepolymer in a reliable and reproducible manner to produce pure difunctional monomers in high yields is questionable [see J. Buck, J. Polym. Sci., Polym. Chem. Ed., 16, 2475-2507 (1978), and U.S. Pat. Nos. 3,975,422, 3,903,055, 4,003,942, 4,012,402, and 4,013,703].
A variety of other processes for producing cyanoacrylate monomers are known, and some of which described below.
U.S. Pat. No. 5,703,267 defines a process for producing a 2-cyanoacrylic acid which comprises subjecting a 2-cyanoacrylate and an organic acid to a transesterification reaction.
U.S. Pat. No. 5,455,369 defines an improvement in a process for preparing methyl cyanoacrylate, in which methyl cyanoacetate is reacted with formaldehyde to form a polymer that is then depolymerized to the monomeric product, and in which the purity of yield is 96% or better. The improvement of the '369 patent is reported to be conducting the process in a poly(ethylene glycol) diacetate, dipropionate, or dibutyrate, having a number average molecular weight of 200-400, as the solvent.
U.S. Pat. No. 6,096,848 defines a process for the production of a biscyanoacrylate, which comprises the steps of esterifying a 2-cyanoacrylic acid or transesterifying an alkyl ester thereof to obtain a reaction mixture; and fractionally crystallizing the reaction mixture to obtain the biscyanoacrylate.
U.S. Pat. No. 4,587,059 defines a process for the preparation of monomeric 2-cyanoacrylates comprising the steps of (a) reacting (i) a 2,4-dicyanoglutarate with (ii) formaldehyde, cyclic or linear polymers of formaldehyde, or a mixture thereof, in the presence of between about 0.5 and about 5 mols of water per mol of 2,4-dicyanoglutarate, at an acid pH of about 3 to slightly less than 7, and at a temperature of about 70 about 140, to form an oligomeric intermediate product, and (b) removing water that is present from step (a) and thermolyzing the oligomeric intermediate product for a period of time sufficient to effect its conversion to monomeric 2-cyanoacrylates.
Commercial production of cyanoacrylate monomers ordinarily relies on the depolymerisation of a prepolymer formed under Knoevenagel condensation reaction conditions, as noted above. Still today the Knoevenagel condensation reaction is believed to remain the most efficient and prevalent commercial method for producing high yields of monofunctional cyanoacrylates. Nevertheless, it would be desirable to not have to resort to thermally induced depolymerisation of a prepolymer produced by the Knoevenagel condensation reaction. This prospect may also enable facile access to highly useful difunctional monomers, such as so-called bis-cyanaocrylates or hybrid materials of cyanoacrylate and other polymerisable or reactive functionality.
The Knoevenagel condensation reaction is well known not only for its usefulness in the manufacture of cyanoacrylates, but also (and perhaps more so) for its immense potential generally in the synthesis of electrophilic olefins from active methylene and carbonyl compounds [G. Jones, Organic Reactions, Vol. XV, 204, Wiley, New York (1967)]. A wide range of catalysts have been employed in carrying out this reaction, each affording variable yields of olefins. More recently, heterogeneous catalysts have been used, many of which include inorganic materials such as clays and zeolites [F. Bigi et al., J. Org. Chem., 64, 1033 (1999)]. These materials are environmentally benign and have been used because they are ditopic in nature, some containing both acidic and basic sites, while others are solely acidic or solely basic. Some Lewis acid catalysts have also been employed in the Knoevenagel condensation reaction [B. Green et al., J. Org. Chem., 50, 640 (1985); P. Rao et al., Tett. Lett., 32, 5821 (1991)]. Since bases are generally active nucleophiles and cyanoacrylate monomers are highly susceptible to initiation of polymerisation by active nucleophiles, it is not possible to exploit base catalysed Knoevenagel synthesis of cyanoacrylate monomer without polymerisation occurring. And while acid catalysed Knoevenagel condensation reactions to form cyanopentadienoate monomers (related to cyanoacrylates) are known (see e.g. U.S. Pat. No. 6,291,544), these routes do not lead to the direct synthesis of cyanoacrylate monomers per se.
The Knoevenagel condensation reaction also liberates water during the condensation of aldehydes with reactive methylene compounds, and neutral and basic water is well known to initiate polymerisation of cyanoacrylate monomers.
Recently, J. S. Yadav et al., “Phosphane-Catalyzed Knoevenagel Condensation: A Facile Synthesis of α-Cyanoacrylates and α-Cyanonitriles”, Eur. J. Org. Chem., 546-551 (2004) reports of the use of triphenylphosphane [sic, triphenylphosphine] as a catalyst for the Knoevenagel condensation of aldehydes with acidic methylene compounds, such as ethyl cyanoacetate and malonitrile, to afford substituted olefins in an efficient manner under mild and solvent-free conditions. Triphenyphosphine is a known initiator for the polymerization of 2-cyanoacrylates.
Absent from the published literature is the use of iminium salts in the preparation of electron deficient olefins, such as 2-cyanoacrylates.
Iminium salts are salts of imines that are in turn reaction products of carbonyl containing compounds and amines. General methods of simple imine formation are described for instance in R. J. Vijin et al., Synthesis, 573 (1994) and U.S. Pat. Nos. 2,582,128 and 5,744,642.
In International Patent Publication No. WO 2003/06225 A1 assigned to BASF Aktiengesellschaft, Lugwigshafen, Germany, ionic liquid precursors such as methyl imidazole are used as solvents and acid scavengers in specific reactions that are known to expel acid byproducts. When these ionic liquid precursors become protonated by the acid expelled during a chemical reaction, such as an esterification of an acid chloride for example, a liquid salt (methyl imidiazolium hydrochloride, for example) forms and subsequently phase separates from the reaction products. That phase separated liquid salt is readily removable and may be treated by base to recycle the original ionic liquid precursor scavenger (methyl imidiazole). The ease of removability of a liquid salt byproducts and the recylability of the byproduct makes this an attractive and important commercial process (so-called BASIL™ process, Biphasic Acid Scavenging Ionic Liquid).
Standard practice to those skilled in the art of organic chemistry is the use of liquid amines, such as triethylamine, as acid scavengers to produce solid quaternary salts. However, these solid quaternary salts are oftentimes difficult and time consuming to separate off from the reaction mixture and are known to cause stirring problems due to thickening of the reaction mixtures, particularly on large reaction scales.